jenkins-community-slave: use requests library to download slave.jar

Do not mix two different HTTP libraries in one script. This fixes a
warning:

/var/lib/jenkins/slave.py:82: DeprecationWarning: URLopener style of invoking requests is deprecated. Use newer urlopen functions/methods

This simple implementation has the disadvantage that it stores the whole
file in RAM, but this isn't an issue for the 1.5MB slave.jar.